In the construction of semiconductor chip package assemblies, it has been found desirable to interpose encapsulant material between and/or around elements of the semiconductor packages in an effort to reduce and/or redistribute the strain and stress on the connections between the semiconductor chip and a supporting circuitized substrate during operation of the chip, and to seal the elements against corrosion, as well as to insure intimate contact between the encapsulant, the semiconductor die and the other elements of the chip package.
Certain designs and methods heretofore have interposed encapsulant materials between semiconductor package elements. Examples of such encapsulants can be found in the aforementioned U.S. Pat. No. 5,659,952 and in the following commonly assigned U.S. patent applications: Ser. No. 08/610,610 filed on Mar. 7, 1996 and Ser. No. 08/726,697 filed on Oct. 7, 1996. Both of such patent applications and their complete disclosures incorporated by reference herein. A further encapsulating method and resulting package structure is shown and described in U.S. Pat. No. 5,148,266, the complete disclosure of which is also incorporated by reference herein.
Other encapsulating schemes have also been used in non-packaging areas, such as the use of a typically epoxy underfill in flip-chip (or "C4") applications to flow around the solderball connections between a semiconductor chip and a substrate. This is referred to as a non-packaging application because the finished assembly is not testable until it is attached to the substrate (such as a PWB) and underfilled; and further, because the flip chip scheme does not allow for the standardization of the electrical terminals which are attached to the substrate.
One such flip chip encapsulation scheme is described in U.S. Pat. No. 3,811,183. This patent refers to a face-down flip-chip type structure soldered to a substrate which is mechanically reinforced by introducing a liquid in the aperture between the semiconductor chip and the substrate and hardening the liquid. The disclosure explains that the drawing of the liquid through the aperture occurs through capillary action due to the small dimension of the aperture which is preferably chosen to be between 1 micron and 100 microns. Another flip chip encapsulation scheme is described in U.S. Pat. No. 5,203,076. This patent describes the need for uniform fillets on all sides of a semiconductor chip which is flip-chip bonded to a substrate because a non-symmetric fillets produce stress gradients in the resulting structure which lead to premature failures of the device. This patent states that dispensing on only two-sides of the flip chip device and relying on capillary action to "suck" a polymer underfill between the chip and the substrate incurs an excessively long cycle time (from 5 to 20 minutes) and creates the aforementioned undesired, non-uniform fillet. The patent sets forth a method to remedy this situation by sealing the entire periphery of the flip chip to the substrate with a bead of underfill material such that the sides of the flip chip make contact with the bead trapping air beneath the flip chip between the flip chip and the substrate. A vacuum chamber is then positioned over the chip, the substrate and the underfill material. Vacuum is continuously applied and entrapped air that was in the underchip area is evacuated by bubbling through the bead of underfill material. Air is then allowed into the vacuum chamber at a controlled rate which forces the underfill material into the underchip area such that there are no voids in the underfill in the underchip area. However, this method is not preferred because the violent bubbling of the entrapped air through the underfill material causes the underfill material to splatter onto adjacent surfaces. One such surface that will be almost inevitably contaminated with the underfill material is the back surface of the flip chip or semiconductor die. The splattering of the underfill material can cause time-consuming and expensive rework and cleaning of the exposed portions of the flip chip device and the adjacent surfaces of the substrate and other components. This problem is particularly severe if the splattered underfill material is not sufficiently cleaned prior to curing the underfill such that the underfill material hardens on undesired surfaces.
Thus, despite all of the effort which has been devoted to development of microelectronic encapsulation techniques (including the positive results of the aforementioned commonly owned inventions), there are unmet needs for further improvements.